The present invention relates to an image heating device.
The image heating device includes a fixing device for heating and fixing an unfixed image on a recording material into a permanent fixed image thereon, an apparatus for temporarily fixing by heating an unfixed image, an apparatus for heating an image to improve a surface property such as gloss, or the like.
a) Heat roller type fixing device
Heretofore, in an image forming apparatus such as a copying machine, a facsimile machine, a laser beam printer or the like using an electrophotographic process, electrostatic recording process or the like, the use is made with a fixing device of a heat roller type which is one of a fixing devices an in which an unfixed toner image formed on a recording material (transfer material, photosensitive paper, electrostatic recording paper, print sheet OHP (OHT) film or the like) is heated and fixed thereon into a permanent fixed image.
FIG. 14 is a schematic cross-sectional view of an example of a heat roller type fixing device.
The fixing device comprises a pair of pressing rollers 201, 202, one of or both of which contain heat sources 201a, 202a such as halogen lamp or the like. The rollers constitute a nip N therebetween, and the recording material P is nipped and fed by the nip while being subjected to heat and pressure, so that unfixed toner image t is fixed on the recording material P.
The fixing roller 201 and the pressing roller 202 comprise hollow core shafts 201b, 202b of aluminum or the like, elastic layers 201c, 202c of silicone rubber on the outer surfaces of the core shafts, and parting layers 201d, 202d of fluorine resin material or the like on the outer surfaces, respectively.
Generally, the roller contactable to the unfixed toner image t is xe2x80x9cfixing rollerxe2x80x9d, and the roller not contactable to the unfixed toner image t is xe2x80x9cpressing rollerxe2x80x9d. The temperatures of the fixing and pressing rollers 201, 202 are detected by temperature detecting means 26 such as thermister means contacted to the roller surface, and the detected temperature is fed back to an ON/OFF control circuit (unshown) for activating and deactivating heat sources 201a, 202a so as to maintain a predetermined temperature.
The heat quantity applied to the recording material P per unit time is determined mainly on the basis of the width of the nip, the roller temperature and the recording material feeding speed. Among these factors, the roller temperature and the recording material feeding speed can be relatively easily exchanged in accordance with a desired fixing process and the recording material so as to supply proper heat quantity.
For example, the glossiness (gloss) of the fixed image is significantly influenced by the recording material feeding speed and the roller control temperature of the fixing device. The gloss tends to be higher if the roller control temperature is higher and if the recording material feeding speed is slower, within a certain range. In other words, the gloss rises with increase of the heat quantity.
By adjusting the parameters, the gloss of the image can be controlled. For example, in an image forming apparatus producing low gloss images, high gloss images can be provided by controlling the fixing condition.
As for the recording material used with the image forming apparatus, there are OHP (overhead projector) film, gloss film or the like in addition to the sheet of paper. The former is a transparent resin film through which light from a projector is transmitted, and the latter is a white resin film having a glossiness. They are made of synthetic resin material film such as PET having a thickness of 4-5 microns so that thermal capacity is very large as compared with that of usually paper. Therefore, a larger amount of heat supply is required to properly fix the unfixed toner image. Furthermore, the OHP film requires good permeability, and gloss film requires high glossiness. In order to accomplish this, it is desirable that toner is sufficiently fused and deformed to smooth the surface of the toner image. Therefore, it is necessary to increase the amount of the heat supply by decreasing the recording material feeding speed or by rising the roller control temperature.
Thus, in order to control the glossiness of the image or to improve the permeability or the glossiness of the image on the film, the recording material feeding speed and/or the roller control temperature is instantaneously switched to provide an optimum amount of the heat supply. If the roller control temperature is constant irrespective of the recording material feeding speed, the amount of the heat tends to be insufficient or too much at the time of changing the recording material feeding speed with the result of fixing defect or unintended image quality. Therefore, it is desirable that roller control temperature is changed in accordance with the recording material feeding speed.
However, in the case of the heat roller type fixing device, the surface temperature of the fixing roller 201 (heating portion material) does not become the set control temperature instantaneously after the roller control temperature is switched. Such a poor thermal responsivity results mainly from the following two factors.
The first is that heat source 201a is away from the surface of the roller in the case of the fixing roller 201. For example, the heat from the heat source 201a in the fixing roller 201 is transferred to the surface of the roller from the heat source 201a (halogen lamp or the like) via air layer, core shaft 201b (A1 or the like), elastic layer 201c the magnetic field for heating an image on a recording material; a temperature detecting element for detecting a temperature of said heating portion material; control means for controlling said magnetic field generating means to maintain a temperature detected by said temperature detecting means at a target temperature; heating condition setting means for setting a feeding speed of the target temperature.
The second cause is that thermal capacity of the entire fixing roller 101 is relatively large. This leads to larger amount of heat quantity required to rise the temperature of the roller so that in the thermal responsivity is worsened. This is also a cause of the fact that roller temperature does not easily lower.
For these reasons, the temperature of the roller surface does not linearly follow the switching of the control temperature.
In such a fixing device, when the recording material feeding speed is switched from a normal speed to a slower speed, excessive supply of the heat results, so that problem of hot offset or OHPpermeability decrease arises. Additionally, there arises a problem that high gloss image is not provided even under a high gloss fixing condition.
Moreover, the next printing operation is required to be interrupted until the surface temperature of the fixing roller 201 reaches the control temperature to avoid the image defects described above.
b) Fixing device of a film heating type using a ceramic heater
The inventors have investigated the possibility of improving the thermal responsivity of a fixing device of a film heating type using a ceramic heater.
Such a fixing device has been proposed in Japanese Laid-open Patent Application No. SHO 63-313182, Japanese Laid-open Patent Application No. HEI 2-157878, Japanese Laid-open Patent Application No. HEI 4-44075, Japanese Laid-open Patent Application No. HEI 4-204980, for example.
FIG. 15 is a schematic view of an example of such a fixing device.
The rotatable member 10 constituting the nip N is a cylindrical fixing film. From the standpoint of reducing the thermal capacity an improve the quick start feature, the fixing film 10 preferably has a film thickness of not more than 100 xcexcm, preferably not more than 50 xcexcm and not less than 20 xcexcm and is a heat resistive film of a monolayer of PTFE, PFA, FEP resin material or a complex layer film comprising a PI, PAI, PSEK, PES, PPS resin material, a coating layer of PTFE, PFA, FEP with an electroconductive primer layer therebetween.
Designated by a reference numeral 16 is a film guide of arcuate trough type.
Designated by a reference numeral 5 is a ceramic heater extending in a longitudinal direction of the film guide nip. The ceramic heater 5 comprises a substrate 5a of alumina or the like, a heat generation layer 5b of Ag/Pd or the like which is painted by screen printing or the like into approx 10 xcexcm thick and 1-5 mm width on the substrate 5a, and a protection layer 5c thereon of glass, fluorine resin material or the like.
Designated by a reference numeral 30 is a pressing roller (pressing rotatable member).
Designated by 26 is a temperature detecting element using a thermister end is disposed on a back side of the ceramic heater 5.
The temperature control is effected by phase control, wave number control or the like of the ACvoltage supplied to the ceramic heater 5 by a TRIAC 6 on the basis of information from the temperature detecting element 26 so that electric power supplied to the ceramic heater 5 is controlled.
A fixing film 10 is sandwiched between the ceramic heater 5 and the pressing roller 30 (pressing member) to form a nip N, and a recording material P carrying an unfixed toner image t is passed through the nip between the fixing film 10 and pressing roller 30, wherein the recording material P is moved together with the fixing film 10, by which the heat from the ceramic heater 5 is applied to an image through the fixing, so that image is fixed on a recording material P by the pressure of the nip N and the heat.
Therefore, the fixing device uses a ceramic heater 5 and fixing film 10 having low thermal capacities to constitute an on-demand type apparatus. Only during the execution of the image forming operation, the electric energy is supplied from the heat source to the ceramic heater 5 to heat it to a predetermined fixing temperature. Therefore, the waiting period until the image formation executable state is reached from the actuation of the power source is short (quick start feature), and the electric energy consumption being the stand-by state is significantly reduced (electric power saving), and therefore, the thermal responsivity is remarkably better than the heat roller type.
The fixing device has a very good thermal responsivity as compared with the heat roller type because the thermal capacity of the film 10 of the fixing member is very small, and because the ceramic heater 5 (heat source) is close to the inner surface of the fixing film 10. Thus, it is possible to cause the surface temperature of the heating portion material (fixing film 10) to follow linearly the switching of the control temperature in response to switching of the recording material feeding speed.
However, such a film heating type fixing device using a ceramic heater having a very good thermal responsivity as compared with the best roller type still involves the following problems.
In the fixing device, the fixing film 10 cannot accommodate the height of the toner image t due to the difference of the quantity of the toner and the unsmoothness of the surface of the recording material itself in some cases, with the result of unable glossiness of the fixed image.
In order to solve this problem, the surface of the fixing film 10 is made soft by a providing a 300 xcexcm thick of an elastic layer of silicone rubber or the like with the fixing film 10. However, when such a fixing device is used as a fixing device for a full color image forming apparatus, the total thermal capacity of the fixing film 10 and the total thermal resistance increase with the result of lowered thermal responsivity of the fixing device.
In a film heating type device using a ceramic heater 5, the pressing roller 30 is urged toward the ceramic heater 5 which is a heat source, through the fixing film 10 therebetween. The heat generation of the ceramic heater 5 results in the thermal-expansion of the heater per se. Therefore, the stress due to the thermal-expansion in the ceramic heater 5 is larger, and therefore, the heater is more easily broken, if the pressure at the nip N is larger.
For this reason, the pressure is not so high in the fixing device of this type. For example, the pressure in the heat roller type can be as high as 40 kgf, whereas in the fixing device of the film heating type using the ceramic heater is approx 10-15 kgf.
Therefore, even if the ORFfilm is fed at a speed lower than the normal speed, the surface of the fixing toner image is not sufficiently smooth due to insufficiency in the pressure with the result of lower permeability of the full-color image on the OHFfilm.
For the same reason, it is difficult to increase the glossiness of the image.
A fixing device using an induction heating technique has been proposed, but no fixing device of the induction heating type capable of selecting optimum fixing condition or a fixing device of the induction heating type capable of selecting the glossiness of the image as desired.
Accordingly, it is a principal object of the present invention to provide an image heating device which is capable of quickly setting an optimum fixing condition in accordance with the recording materials.
It is another object of the present invention to provide an image heating device which is capable of setting the glossiness of the image as desired.
According to an aspect of the present invention, there is provided an image heating apparatus, comprising magnetic field generating means for generating a magnetic field; a heating portion material for generating heat using eddy currents produced by themagnetic field and for heating an image on a recording material; a temperature detecting element for detecting a temperature of said heating portion material; control means for controlling said magnetic field generating means to maintain a temperature detected by said temperature detecting means at a target temperature; heating condition setting means for setting a feeding speed of the target temperature.
According to another aspect of the present invention, there is provided an image heating apparatus, comprising: a magnetic field generating means for generating a magnetic field; a heating portion material for generating heat using eddy currents produced by the magnetic field and for heating an image on a recording material; a temperature detecting element for detecting a temperature of said heating portion material; control means for controlling said magnetic field generating means to maintain a temperature detected by said temperature detected by said temperature detecting means at a target temperature; wherein the target temperature is variable.